It is widely known that a transistor draws very little power when it is operating fully switched “on” or fully switched “off.” Small, battery-operated audio power amplifiers, such as those found in MP3 players, and hearing aids, require this efficiency to obtain sufficient battery life, small size, lower weight, and reduced cost.
An analog power amplifier is usually operating between these two extremes and wastes power by creating wasted heat. To solve this problem, a switching amplifier, or “class-D” amplifier is used. The switching amplifier operates in on/off mode, which generates a series of pulses whose average value is proportional to the amplitude of the input signal at that time. The output passes through a low pass filter to remove high frequency switching noise.
The most common way to switch the power amplifier “on” and “off” is by using a pulse width modulator, as shown in FIG. 1. A typical pulse width modulator 12 includes a ramp generator 14 to generate a saw-tooth or triangular waveform that is compared to the input analog signal 16 at a comparator 18. When the input is greater than the ramp generator output the power amplifier is turned “on,” otherwise the amplifier is turned “off.” The result is a series of high frequency pulses whose width varies with the input signal. When the input voltage is high, the pulses are very wide, whereas when the input is low, they are very narrow. There is a substantial amount of electromagnetic interference (EMI) generated at the pulse repetition frequency, which is equal to the frequency of the ramp generator. The EMI must be shielded from external circuits or devices in the vicinity.
A power transistor in a class D amplifier is relatively inefficient during the time that it switches from “on” to “off” or vice versa. During each cycle of the ramp oscillator the transistor switches twice: once to “on” and once to “off.” When using a pulse width modulator, the higher the oscillator frequency, the higher the switching rate, and the higher the losses.
U.S. Pat. No. 6,888,403 entitled “PULSE WIDTH MODULATION TYPE AUDIO AMPLIFIER HAVING VARIABLE AVERAGE SWITCHING FREQUENCY” discloses a method of mitigating these losses by using a bandwidth detector to determine the minimum possible switching rate. As shown in FIG. 2, the frequency of the ramp generator is controlled by a frequency controller 22, which uses a bandwidth detector 24 to select an appropriate switching rate. For low-bandwidth audio signals, like bass signals for a subwoofer, a low switching rate is used. The circuit shown in FIG. 2 is not power efficient because the entire circuit is active all the time, even when no input signal is present. The circuit also produces electromagnetic interference that is concentrated into a very narrow frequency range and thus much harder to shield. The present invention is not lacking in these regards. U.S. Pat. No. 6,888,403 is hereby incorporated by reference into the specification of the present invention.
There exists a need for a simple, easy to construct, efficient power amplifier that reduces electromagnetic interference.